The limits on how many agents a collector can serve is more a case of how many metrics can the collector handle as that is far more relevant. The default maximum number of agents per collector is defined as 400 in apm-events-thresholds-config.xml
<clamp id="introscope.enterprisemanager.agent.connection.limit">
<description>
Limits the # of agent connections
</description>
<threshold value="400"/>
this is also where the default max number of live metrics is defined
<clamp id="introscope.enterprisemanager.metrics.live.limit">
<description>
Per EM limit. Takes into account only live metrics (i.e. currently reporting from Agents)
</description>
<threshold value="500000"/>
A collector is just a specific type of EM so the answer is the same for both.
As for what the maximum number that can be handled that will very much depend on the specification of the system used and I would recommend reviewing the sizing guide APM Sizing and Performance Guide - CA Application Performance Management - 10.3 - CA Technologies Documentation
thanks
Mike